1. Field of the Invention
This invention relates to a dry etching method adopted for production of semiconductor devices, particularly to a method of etching a silicon oxide layer while securing high selectivity to a silicon nitride based material layer.
2. Description of the Related Art
As semiconductor devices of large-scale integration and high performance have been diffused, as seen in VLSI and ULSI of recent years, a technique of dry etching for an insulation film is largely demanded for achieving high anisotropy, high etchrate, high selectivity, low damage and low pollution without sacrificing any one of these requirements.
For etching an insulation film consisting of silicon oxide (SiO.sub.x, where x=2 in particular), CHF.sub.3, CF.sub.4 /H.sub.2 mixed, CF.sub.4 /O.sub.2 mixed, and C.sub.2 F.sub.6 /CHF.sub.3 mixed gases have hitherto been used as etching gases. These gases consist mainly of a fluorocarbon based gas with a ratio of the number of carbon atoms to the number of fluorine atoms in a molecule or C/F ratio of 0.25 or higher. These gas systems are used for the following reasons: (a) C contained in the fluorocarbon based gas forms a C--O bond on an SiO.sub.2 layer surface, thereby cutting or weakening the Si--O bond; (b) CF.sub.n.sup.+ (particularly n=3) as an etchant for the SiO.sub.2 layer; and (c) since a relatively carbon-abundant state is created in a plasma, oxygen within SiO.sub.2 is removed in the form of CO or CO.sub.2 while a deposition of a carbonaceous polymer on the surface of a underlying silicon due to a contribution of C, H and F contained in the gas lowers the etchrate, thereby securing high selectivity to the underlying layer.
The above-mentioned additional gases such as H.sub.2, O.sub.2, etc., which are used for the purpose of controlling selection ratio, can reduce or increase the amount of F* generation. In short, these additional gases exhibit effects of controlling the apparent C/F ratio in the etching reaction system.
Fundamentally, an insulation film consisting of silicon nitride (Si.sub.x N.sub.y, where x=3, y=4 in particular) also is etched with the same gas system as in the case of etching the SiO.sub.2 layer. However, while the SiO.sub.2 layer is etched by a mechanism primarily of ion-assisted reaction, the Si.sub.x N.sub.y layer is etched on the basis of a radical reaction mechanism using F* as the main etchant, with higher etchrate than in the case of the SiO.sub.2 layer. This can be more or less predicted from the following relation in the amount of inter-atomic bond energies: Si--F bond (132 kcal/mole) &gt;Si--O bond (111 kcal/mole) &gt;Si--N bond (105 kcal/mole). Different values of inter-atomic bond energies may be given, depending on calculation methods. However, data described in R. C. Weast ed., "Handbook of Chemistry and Physics," 69th ed., 1988, CRC Press, Florida, USA, are referred to here.
Meanwhile, as the device structure has become complex recently, it has become necessary to carry out etching highly selective between the SiO.sub.x layer and the Si.sub.x N.sub.y layer.
For example, etching of the Si.sub.x N.sub.y layer on the SiO.sub.x layer is carried out with patterning for prescribing an element isolation region in the LOCOS method. In the present state of art wherein a pad oxide film (SiO.sub.2 layer) is thinned for minimizing the bird's beak length, the above-mentioned etching is a process requiring extremely high selectivity to the underlying layer.
On the other hand, etching of the SiO.sub.x layer on an SiN.sub.x is required in, for example, a contact hole processing. There are some cases in recent years in which a thin Si.sub.x N.sub.y layer is provided under an SiO.sub.x interlayer insulation film for the purpose of reducing damages to a wafer at the time of overetching. In these cases, also, high selectivity to the underlying layer is demanded for attaining the purpose.
However, in the case of using an SiO.sub.x layer and an Si.sub.x N.sub.y layer, values of inter-atomic bond energies of an Si--O bond and an Si--N bond are close to each other, and a common etching gas is used. Therefore, highly selective etching is essentially difficult. Techniques for enabling the highly selective etching have hitherto been developed in various places.
There are some reports on the techniques for etching the Si.sub.x N.sub.y layer on the SiO.sub.x layer.
For instance, the present inventor previously disclosed a technique using an etching gas consisting of CH.sub.2 F.sub.2 gas of a lower C/F ratio (ratio of the number of C atoms to the number of F atoms in one molecule) mixed with CO.sub.2 in a mole ratio of 30 to 70%, in the Japanese Patent KOKAI publication Serial No. 61-142744. The gas with a small C/F ratio can form CF.sub.x.sup.+ (particularly x=3) as an etchant for the SiO.sub.x layer only by rebonding of F*. However, if a great amount of CO* is supplied to this gas system so as to capture F* for removal in the form of COF, the amount of CF.sub.x.sup.+ generation is reduced, thus lowering the etchrate for etching the SiO.sub.2 layer. On the other hand, Si.sub.x N.sub.y is etched by ions and radicals other than CF.sub.x.sup.+, the addition of a large amount of CO.sub.2 does not change the etchrate. In this manner, selectivity between both layers can be achieved.
Also, a technique for etching an Si.sub.x N.sub.y layer on an SiO.sub.x layer, by supplying NF.sub.3 and Cl.sub.2 to a chemical dry etcher and by utilizing FCl formed in a gaseous phase by microwave discharges, is reported in Proceedings of Symposium on Dry Process, vol. 88, No. 7, 1987, pp. 86 to 94. While the Si--O bond contains ion bond by 55%, the Si--N bond contains ion bond by 30%, having higher covalent bond. In short, characteristics of chemical bond in the Si.sub.x N.sub.y layer are close to those of chemical bond (covalent bond) in single-crystal silicon, and thus the Si.sub.x N.sub.y layer is etched by radicals, such as F* and Cl* dissociated from FCl. On the other hand, the SiO.sub.x layer is hardly etched by these radicals, thus enabling highly selective etching.
In this manner, there have been several reports on the techniques for selective etching of the Si.sub.x N.sub.y layer on the SiO.sub.x layer. These techniques are a matter of course in a sense, if the etchrates for the two layers are taken into account. That is, if the SiO.sub.x layer is exposed in the middle of the process of etching the Si.sub.x N.sub.y layer by the mechanism primarily of radical reactions, the etchrate is necessarily lowered.
However, these conventional techniques have drawbacks. For instance, in the above-mentioned process using FCl, the use of radical reactions renders it essentially difficult to carry out anisotropic processing.
On the contrary, there has been no report on the technique for selective etching of an SiO.sub.x layer on an Si.sub.x N.sub.y layer. In this case, even when the SiO.sub.x layer is etched by the mechanism primarily of ion-assisted reactions, radicals are necessarily formed in the reaction system, raising the etchrate at the time exposition of Si.sub.x N.sub.y. Thus, it is much more difficult to secure selectivity. However, this selective etching is a process to be demanded in the future, and the realization thereof is desired.